Acids and Bases: A Brief Review

Transcription

1 Acids and : A Brief Review Acids: taste sour and cause dyes to change color. : taste bitter and feel soapy. Arrhenius: acids increase [H ] bases increase [OH ] in solution. Arrhenius: acid base salt water. Problem: the definition confines us to aqueous solution. BrønstedLowry Acids and The H Ion in Water The H (aq) ion is simply a proton with no electrons. (H has one proton, one electron, and no neutrons.) In water, the H (aq) form clusters. The simplest cluster is H 3 O (aq). Larger clusters are H 5 O 2 and H 9 O 4. Generally we use H (aq) and H 3 O (aq) interchangeably. BrønstedLowry Acids and Proton Transfer Reactions Focus on the H (aq). BrønstedLowry: acid donates H and base accepts H. BrønstedLowry base does not need to contain OH. Consider HCl(aq) H 2 O(l) H 3 O (aq) Cl (aq): HCl donates a proton to water. Therefore, HCl is an acid. H 2 O accepts a proton from HCl. Therefore, H 2 O is a base. Water can behave as either an acid or a base. Amphoteric substances can behave as acids and bases. BrønstedLowry Acids and Conjugate AcidBase Pairs Whatever is left of the acid after the proton is donated is called its conjugate base. Similarly, whatever remains of the base after it accepts a proton is called a conjugate acid. Consider HA(aq) H 2 O(l) H 3 O (aq) A (aq) After HA (acid) loses its proton it is converted into A (base). Therefore HA and A are conjugate acidbase pairs. After H 2 O (base) gains a proton it is converted into H 3 O (acid). Therefore, H 2 O and H 3 O are conjugate acidbase pairs. Conjugate acidbase pairs differ by only one proton. BrønstedLowry Acids and Relative Strengths of Acids and The stronger the acid, the weaker the conjugate base. H is the strongest acid that can exist in equilibrium in aqueous solution. OH is the strongest base that can exist in equilibrium in aqueous solution. 1

2 BrønstedLowry Acids and Relative Strengths of Acids and Any acid or base that is stronger than H or OH simply reacts stoichiometrically to produce H and OH. The conjugate base of a strong acid (e.g. Cl ) has negligible acidbase properties. Similarly, the conjugate acid of a strong base has negligible acidbase properties. The Autoionization of Water The Ion Product of Water In pure water the following equilibrium is established H 2 O(l) H 2 O(l) H 3 O (aq) OH (aq) at 25 C [H 3O ][OH ] Keq = [H 2 2O] K [H 2 2O] = [H3O eq ][OH ] K [H 14 3O ][OH ] w = = The above is called the autoionization of water. The ph Scale In most solutions [H (aq)] is quite small. We define ph = log[h O ] log[h ] poh log[oh 3 = = ] In neutral water at 25 C, ph = poh = In acidic solutions, [H ] > , so ph < In basic solutions, [H ] < , so ph > The higher the ph, the lower the poh, the more basic the solution. The ph Scale Most ph and poh values fall between 0 and 14. There are no theoretical limits on the values of ph or poh. (e.g. ph of 2.0 M HCl is ) 2

3 The ph Scale Other p Scales In general for a number X, px = logx For example, pk w = log K w. K = [H ][OH ] = w pk log[h ( ][OH w = ]) = 14 log[h ] log[oh ] = 14 ph poh = 14 The ph Scale Measuring ph Most accurate method to measure ph is to use a ph meter. However, certain dyes change color as ph changes. These are indicators. Indicators are less precise than ph meters. Many indicators do not have a sharp color change as a function of ph. Most indicators tend to be red in more acidic solutions. The ph Scale Strong Acids and Strong Acids The strongest common acids are HCl, HBr, HI, HNO 3, HClO 3, HClO 4, and H 2 SO 4. Strong acids are strong electrolytes. All strong acids ionize completely in solution: HNO 3 (aq) H 2 O(l) H 3 O (aq) NO 3 (aq) Since H and H 3 O are used interchangeably, we write HNO 3 (aq) H (aq) NO 3 (aq) Strong Acids and Strong Acids In solutions the strong acid is usually the only source of H. (If the molarity of the acid is less than 10 6 M then the autoionization of water needs to be taken into account.) Therefore, the ph of the solution is the initial molarity of the acid. Strong Most ionic hydroxides are strong bases (e.g. NaOH, KOH, and Ca(OH) 2 ). Strong Acids and Strong Strong bases are strong electrolytes and dissociate completely in solution. The poh (and hence ph) of a strong base is given by the initial molarity of the base. Be careful of stoichiometry. In order for a hydroxide to be a base, it must be soluble. do not have to contain the OH ion: O 2 (aq) H 2 O(l) 2OH (aq) H (aq) H 2 O(l) H 2 (g) OH (aq) N 3 (aq) H 2 O(l) NH 3 (aq) 3OH (aq) 3

4 Weak acids are only partially ionized in solution. There is a mixture of ions and unionized acid in solution. Therefore, weak acids are in equilibrium: HA(aq) H 2 O(l) H 3 O (aq) A (aq) [H O ][A 3 ] K a = [HA] HA(aq) H (aq) A (aq) [H ][A ] K a = [HA] K a is the acid dissociation constant. Note [H 2 O] is omitted from the K a expression. (H 2 O is a pure liquid.) The larger the K a the stronger the acid (i.e. the more ions are present at equilibrium relative to unionized molecules). If K a >> 1, then the acid is completely ionized and the acid is a strong acid. Calculating K a from ph Weak acids are simply equilibrium calculations. The ph gives the equilibrium concentration of H. Using K a, the concentration of H (and hence the ph) can be calculated. Write the balanced chemical equation clearly showing the equilibrium. Write the equilibrium expression. Find the value for K a. Write down the initial and equilibrium concentrations for everything except pure water. We usually assume that the change in concentration of H is x. Calculating K a from ph Substitute into the equilibrium constant expression and solve. Remember to turn x into ph if necessary. Using K a to Calculate ph Percent ionization is another method to assess acid strength. For the reaction HA(aq) H 2 O(l) H 3 O (aq) A (aq) [H O 3 ] eqm % ionization= 100 [HA] 0 Using K a to Calculate ph Percent ionization relates the equilibrium H concentration, [H ] eqm, to the initial HA concentration, [HA] 0. The higher percent ionization, the stronger the acid. Percent ionization of a weak acid decreases as the molarity of the solution increases. For acetic acid, 0.05 M solution is 2.0 % ionized whereas a 0.15 M solution is 1.0 % ionized. 4

6 Relationship Between K a and K b AcidBase Properties of Salt Solutions Nearly all salts are strong electrolytes. Therefore, salts exist entirely of ions in solution. Acidbase properties of salts are a consequence of the reaction of their ions in solution. The reaction in which ions produce H or OH in water is called hydrolysis. Anions from weak acids are basic. Anions from strong acids are neutral. AcidBase Properties of Salt Solutions An Anion s Ability to React with Water Anions, X, can be considered conjugate bases from acids, HX. For X comes from a strong acid, then it is neutral. If X comes from a weak acid, then X (aq) H 2 O(l) HX(aq) OH (aq) The ph of the solution can be calculated using equilibrium! AcidBase Properties of Salt Solutions An Cation s Ability to React with Water Polyatomic cations with ionizable protons can be considered conjugate acids of weak bases. NH 4 (aq) H 2 O(l) NH 3 (aq) H 3 O (aq) Some metal ions react in solution to lower ph. Combined Effect of Cation and Anion in Solution An anion from a strong acid has no acidbase properties. An anion that is the conjugate base of a weak acid will cause an increase in ph. AcidBase Properties of Salt Solutions Combined Effect of Cation and Anion in Solution A cation that is the conjugate acid of a weak base will cause a decrease in the ph of the solution. Metal ions will cause a decrease in ph except for the alkali metals and alkaline earth metals. When a solution contains both cations and anions from weak acids and bases, use K a and K b to determine the final ph of the solution. AcidBase Behavior and Factors that Affect Acid Strength Consider HX. For this substance to be an acid we need: HX bond to be polar with Hδ and Xδ (if X is a metal then the bond polarity is Hδ, Xδ and the substance is a base), the HX bond must be weak enough to be broken, the conjugate base, X, must be stable. 6

7 AcidBase Behavior and Binary Acids Acid strength increases across a period and down a group. Conversely, base strength decreases across a period and down a group. HF is a weak acid because the bond energy is high. The electronegativity difference between C and H is so small that the CH bond is nonpolar and CH 4 is neither an acid nor a base. AcidBase Behavior and Binary Acids AcidBase Behavior and Oxyacids Oxyacids contain OH bonds. All oxyacids have the general structure YOH. The strength of the acid depends on Y and the atoms attached to Y. If Y is a metal (low electronegativity), then the substances are bases. If Y has intermediate electronegativity (e.g. I, EN = 2.5), the electrons are between Y and O and the substance is a weak oxyacid. AcidBase Behavior and Oxyacids If Y has a large electronegativity (e.g. Cl, EN = 3.0), the electrons are located closer to Y than O and the OH bond is polarized to lose H. The number of O atoms attached to Y increase the OH bond polarity and the strength of the acid increases (e.g. HOCl is a weaker acid than HClO 2 which is weaker than HClO 3 which is weaker than HClO 4 which is a strong acid). AcidBase Behavior and Oxyacids AcidBase Behavior and Carboxylic Acids Carboxylic acids all contain the COOH group. All carboxylic acids are weak acids. When the carboxylic acid loses a proton, it generate the carboxylate anion, COO. O R C OH 7

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